Rainbow vanilla cake

My son asked for a rainbow cake for his birthday, so I gave it a go and I've never been so proud of a cake. He said I made his dreams come true – I don't think there could be any higher praise.

I used Sally's 6-inch vanilla cake recipe and coloured each layer using gel. The decoration was a white buttercream crumb coat, followed by dotting on and blending each colour separately (chilling for 10 minutes between each colour).

I'm still figuring out icing entire cakes, so tips on how to avoid the air bubble effect whilst smoothing would be welcome!